ABSTRACT The study of human infant immunity has been challenging due to the transient nature of this phase of life, the ethical consideration, and the limited accessibility to critical sites of immune cell development beyond the umbilical and peripheral blood source. One approach for studying the human immune system in vivo is the use of humanized mice. However, while the conventional models (e.g., NSG mice) allow an efficient development of human T and B cells, these models failed to support the development of human NK cells. As a result, exploring human NK cell development and function in vivo remained largely disadvantaged until the recent technological development of human hemato-lymphoid system mice in which the genes of human cytokines were knocked into their respective mouse loci, allowing for a successful development of human NK cells. In this capacity, these human hemato-lymphoid system mice represent the best mouse model currently available to permit the study of human NK cells in vivo. The premise of this proposal is to establish, using these system mice, an animal model for the study of human NK cell ontogeny that could be used for validation and targeted immunotherapies in pediatric diseases. Specifically, the goal of this proposal is to characterize the ontogeny of circulatory and tissue-resident human NK cells (Aim 1), and trace the age-dependent changes of TGF?R pathway during the maturation of human NK cells from early to adult life (Aim 2). At completion of Aim 1, we expect to obtain a map to trace the dynamic and distribution of human precursor NK cells, conventional circulatory NK cells, and tissue-resident NK cells. Currently, there is no available information on genomic profiling of human NK cells during ontogeny, which underscores the value of the data to be generated under Aim 2.